A known process for converting sound waves into digital electrical signals involves the use of an electro-acoustic transducer which is operable to produce electrical signals in dependence on the received sound waves. The signals are then translated into pulse-coded signals. Thus, in one form of such a process, a signal generator converts pressure waves into digital electrical signals, an arrangement of a plurality of sensors being activated by a scanning generator in a suitable sequence. The sensor outputs are applied to a coding matrix which produces binary coded signals. Another form of such a process involves the use of a combination of a microphone and an A-D converter, for converting the sound waves into digital signals. In both these processes, the output signal obtained is in the form of a PCM-signal (pulse code modulation signal).
In comparison, there are also other digital signal forms, for example a difference pulse code modulation signal (DPCM-signal). This gives the difference between two signal portions which occur in succession in time, in the form of a 1-bit signal, For the purposes of producing a DPCM-signal, an A-D converter is required, which is in the form of a storage means for storing a given signal portion until the next following one can be scanned. The first signal portion can then be cancelled or can be over-written with the content of the next following. For this purpose, it is necessary to operate with a high degree of electrical precision and also to use a clock signal for controlling writing into and reading out of the storage device.
When DPCM-signals are converted into PCM-signals, for example by means of a counter, in theory there is no detrimental effect in regard to quality if the following condition is fulfilled: EQU f.sub.DPCM =f.sub.PCM .multidot.n
In the foregoing condition:
n denotes the number of bits per digital word in the PCM-method, PA1 f.sub.PCM is the scanning frequency in the PCM-method, and PA1 f.sub.DPCM is the scanning frequency in the DPCM-method.
With processes which are generally employed at the present time, with 16 bits per word and a scanning frequency f.sub.PCM of 40 kHz, the DPCM scanning rate must therefore be at least 640,000 bits/s. Because of the precision required in regard to the writing and reading operations in respect of the storage means, it will be seen that the band width required for the above-mentioned clock must be substantially higher.